A particularly stressful environment in which distance measuring systems are used is that of motor vehicle applications. Despite inaccuracy caused by vibration and other environmental factors, the need for such devices is substantial, and distance measuring systems are used to increase the safety of motor vehicle operation.
While modern vehicle design provides an operator satisfactory forward visibility during vehicle operation, vision in side and rear directions is often obstructed. To some extent operator vision is augmented by proper placement of mirrors on the vehicle. However, mirrors do not completely eliminate blind spots. For example, it is difficult to provide a satisfactory view toward the rear of a vehicle using mirrors due to obstructions caused by the body of the vehicle. Operator vision toward the rear of the vehicle is further impeded by the relatively large distance from the operator position to the rear of the vehicle common to tractor-trailer combinations and other large vehicles.
Mirrors further limit visual clues necessary for operator judgment of distance to an obstruction by limiting operator depth perception. This is a particular problem when backing a vehicle when the operator requires accurate distance information to maneuver a vehicle (e.g., a semi-tractor trailer) toward an obstruction such as a loading dock. Because of the relatively large distance between the operator and the rear if the vehicle, smaller obstructions may go unnoticed. This is also true in blind areas one either side of a vehicle where direct driver observation is not possible and mirror systems provide incomplete coverage.
Various systems have been proposed in addition to mirrors to augment operator vision. These systems include visual systems using television cameras and monitors to allow visual observation of areas not viewable through mirrors. Television augmentation is relatively extensive, requires the operator to divert his or her attention from other visual systems, and provides limited clues to determine distance to an obstruction.
Electronic obstruction detecting and ranging systems use ultrasonic, radar and infrared emitters and receivers to bounce energy off an obstruction the systems measure the time required for the emitted energy to travel to and from the obstruction to determine a distance to the obstruction.
Starke et al. U.S. Pat. No. 4,903,004, incorporated herein by reference, describes a distance measuring and signalling system for a vehicle for measuring distances to obstructions such as behind a vehicle being backed up. An array of transducers are mounted on a support such as a rear bumper of the vehicle. A pulsed signal is timed as it is converted to an ultrasonic wave signal and its echo detected. The calculated distance is displayed on a three digit display mounted on a dashboard of the vehicle. The system includes ultrasonic transmitting/receiving units, a control unit and a digital display unit.
Tendler U.S. Pat. No. 4,937,796 describes a method and apparatus for providing a voice alert to a vehicle operator. The apparatus uses sonar to detect a distance from the rear of a vehicle to a structure such as a loading dock to which the vehicle is being backed. A device converts a detected range into an audible call out of the distance from the back of the vehicle to the structure.
Naruse U.S. Pat. No. 4,674,073 describes an apparatus for use in a vehicle for ultrasonically locating and ranging an object in relation to the vehicle. A plurality of ultrasonic transmitting and receiving elements are alternately arranged in a linear array and sequentially activated to detect and locate an object. A digital display shows the location and distance to a detected object.
Chey U.S. Pat. No. 4,626,850 describes an apparatus for use in a vehicle for ultrasonically locating and ranging an object in the vicinity of the vehicle. An ultrasonic transducer is rotated in azimuth to scan an area for objects. Audio and visual display of distance and direction to a detected object are provided.
Lee U.S. Pat. No. 4,943,796 describes a display unit attachable to an inside rear view mirror of an automobile for displaying distance to an object behind the vehicle using ultrasonic sensors.
While these publications describe ultrasonic detection and ranging devices, conventional visual distance displays require the vehicle operator to divert attention away from other systems to observe the device display. Although Lee describes a display mounted on an inside mirror so that a distance display is observable near the mirror, the system still requires the vehicle operator to divert his or her vision from the mirror to the attached display. Further, this latter system is not usable with tractor-trailer combinations not having an inside rear view mirror.
The ultrasonic probe signals of the aforementioned devices can be generated using electrostatic transducers. However, these are very sensitive to environmental conditions. Consequently, piezoelectric devices are used instead since they are more robust and less sensitive to environmental effects. However, despite the more robust nature of piezoelectric devices, they display some sensitivity to the environment, and have certain inherent drawbacks. One such drawback is ringing phenomena, i.e., the generation of a false echo due to residual vibrations of a transmitting piezoelectric transducer after transmission of a probe signal. Thus, while piezoelectric transducers are less prone to breakdown than electrostatic transducers, they are subject to false readings due to ringing. These systems are also subject to false readings due too false echoes caused by extraneous environmental factors rather than actual objects meant to be detected. As safety considerations make the use of the aforementioned mirror display desirable, they also mandate a high degree of reliability in the operation of the distance measuring system.